This invention relates generally to a system for regenerating an etchant, and more particularly to an apparatus and method for monitoring and manipulating etchant composition.
Cupric chloride and ferric chloride solutions are commonly used for etching copper and iron, respectively. Ferric chloride may also be used to etch stainless steel and alloys composed of iron and nickel. Although these etchants are quite effective in etching metal from the workpiece, the etching procedure becomes gradually less efficient. The etching process causes a continuous reduction of cupric ions (Cu2+) to cuprous ions (Cu1+), and ferric ions (Fe3+) to ferrous ions (Fe2+). Cuprous ions and ferrous ions are ineffective as etchants and retard the etching procedure as their concentration increases. Thus, the continuous accretion of cuprous or ferrous ions into the etching solution reduces the effectiveness of the etching process over time.
Cupric chloride and ferric chloride change color and clarity as their composition changes. A fresh solution of cupric chloride is a clear green color, but a spent solution, containing a substantial amount of cuprous chloride, turns opaque brown because cuprous chloride, being insoluble, forms a brownish precipitate. A fresh solution of ferric chloride is a clear red-amber color, but a spent solution, containing a substantial amount of ferrous chloride, is clear green.
To compensate for the gradual degradation of etching efficiency, prior art systems became available to regenerate the etchant. One such prior art system is disclosed in U.S. Pat. No. 4,132,585, commonly owned by the assignee of the present invention, the contents of which are hereby incorporated by reference. The ""585 patent discloses an etchant regeneration system that monitors the composition of etchant solution withdrawn from an etcher to diagnose a component deficiency. A light sensor is responsive to the color density of light rays passing through the etchant. A first meter relay is responsive to the light sensor. If the color density of the etchant falls outside preset levels, a pump is energized causing addition of a constituent component into the etchant. A second light sensor senses the color density of the light rays passing through the etchant after the addition of the constituent component. A second meter relay is responsive to the second light sensor. If no improvement of the etchant color is detected, the second meter relay causes the discontinuance of the constituent component addition and switches to the addition of another constituent component, until the etchant is restored.
The conventional light sensor in the prior art system uses an incandescent light bulb for shining light through the wall of an acrylic pipe through which the etchant is flowing. The section of pipe where the light passes through the etchant is referred to as the sensing chamber. The light then passes out from the etchant, through the pipe wall opposite the light source, and strikes a detector. The detector measures the amount of light that penetrated the liquid.
Although effective for regenerating a certain amount of spent cupric chloride, the conventional sensor suffers from several disadvantages. First, the sensing chamber is made of acrylic. The ability of light to penetrate the acrylic varies from spot to spot, and the acrylic becomes cloudy over time. The detector and light source must therefore be calibrated frequently to account for these variables. The acrylic also has the tendency to diffuse the incoming light. Some of the incident light will therefore travel through the acrylic and reach the detector, rather than passing through the etchant flow as intended. Moreover, the acrylic is sensitive to changes in temperature. For example, in cold climates, the sudden change in temperature when the etcher is started may shatter the acrylic.
The size and shape of the sensing chamber in the prior art system also contribute to inaccuracy in the color density measurement. In one embodiment, the sensing chamber is about 15 mm in diameter to promote etchant flow through the regeneration system. Because the light from the light source must cross this distance before striking the detector, much of the light may become blocked in spent solutions. The blocking of incident light contributes to less accurate measurement of the transparency of the etchant except at low etchant concentrations, and limits the maximum concentration that may be measured.
Another disadvantage of the conventional sensor is its use of an incandescent bulb as the source of light. Incandescent, white light covers a wide range of wavelengths. Only specific wavelengths of light, however, are useful for monitoring the etchant in the sensing chamber. To pass a high intensity of light at a specific wavelength through the etchant, a very high intensity of incandescent light must be sent into the chamber. At very high intensity, an incandescent bulb generates so much heat that it damages over time both the acrylic tubing that surrounds the sensing apparatus, and the detecting device. Both types of damage lead to inaccuracy in the sensing system, and to the inefficient regeneration of the etchant.
The high intensity of incident white light also results in inaccurate detector operation. Stray light of wavelengths outside the ideal range may reach the detector to an inconsistent and unpredictable extent. The detector cannot distinguish between the stray light and the light within the range of appropriate wavelengths. To account for this discrepancy, the detectors must be frequently calibrated. However, because of the unpredictability of the amount of extra light reaching the detector, the calibration process is often difficult.
Another consequence of the relatively low intensity of light at suitable wavelengths is the inability of the sensor to operate above relatively low concentrations of metal copper in the etchant, and the inability of the sensor to operate at all with iron etchants. When less light in the specific wavelength band best transmitted by transparent etchant is sent into the liquid, less precipitate is required to completely block the light. This amount of precipitate is the most that can be accurately monitored. Because the amount of precipitate is roughly proportional to the amount of free metal in the solution, the concentration of metal copper is similarly restricted. The maximum concentration of copper that can be sensed in the conventional system is about 27 ounces per gallon of etchant.
Measurement of the transparency of ferric chloride requires the use of light in the red wavelengths. Incandescent bulbs produce only a low intensity of light in this wavelength region. To generate enough red light to monitor a substantial amount of ferric chloride, the incandescent bulb would have to be so bright that the heat generated by the incandescent bulb would destroy the sensor. Also, any green light would make the sensor useless. Therefore, the on-site regeneration of ferric chloride through the use of the conventional sensing system is not feasible. The regeneration of ferric chloride is currently accomplished via the trial-and-error addition of large quantities of acid and oxidizer at an off-site facility. The regenerated etchant is then resold to the etcher. The transportation of the deteriorated etchant to and from the recycling facility, and the measures required to do so safely, incur additional expense for the etcher.
The conventional detector also has various disadvantages. The detector opposite the bulb is a cadmium sulfide photocell, which has a tolerance as high as 200%. As a result, the two detectors in a given apparatus have to be matched to each other to obtain accurate operation of the regeneration system. Frequent recalibration of the detectors is required. Also, the detector is not sealed against contamination by gaseous chlorine or by solvents inside the acrylic pipe. The lack of protection requires frequent replacement of the detector.
The invention provides in one embodiment for a more accurate sensor for monitoring the color density of various etchants. The invention may be used in etchant regeneration machines, which add appropriate amounts of acid and/or oxidizer to the etchant based upon the measurements of the sensor. The etchant regeneration process uses material etched from a workpiece, and added acid and oxidizer to create more etchant. Excess etchant is then removed from the system and recycled.
In a presently preferred embodiment, a sensor for an etchant regeneration system comprises a Pyrex or equivalent tubular sensing chamber and a light cell housing surrounding a portion of the chamber. The sensing chamber may alternatively be made of another transparent heat resistant and durable material. Etchant is diverted into the chamber from a conventional etching system. The chamber contains a rodlike extension extending into the interior of the chamber. The housing accommodates a light source, preferably an LED or laser, and a photodetector, optically coupled through an aperture in the housing.
In alternate embodiments, the invention provides regeneration systems. Two sensors are used in a regeneration apparatus attached to an etching machine used for etching copper, iron, stainless steels or other materials. In another embodiment of the present invention, multiple sensors are used to detect multiple constituents of the etchant depending on the material being etched and the etchant used. A system employing two sets of sensors is particularly applicable to etchants that pass one color of light well when fully regenerated and another color of light well when fully xe2x80x9cspentxe2x80x9d. In additional embodiments, more than two sets of sensors may be used to detect the presence of multiple etchant constituents.